1. Technical Field of the Invention
This invention relates to an improvement of an apparatus and method of forming three-dimensional models from a liquid photopolymer.
2. Prior Art
Rapid Prototyping (RP) evolved to solve the need to accurately, economically, and, most importantly, quickly produce prototype parts and conceptual design models. Designers and engineers can now quickly produce models for early optimization, visualization, and verification. High quality, prototype parts, or models, produced by RP systems provide extremely valuable feedback in the process of product development. While many techniques are currently deployed commercially or are currently in development, the application of lithographic techniques has assumed the leading role. This process, known as stereolithography, forms solid parts from a liquid photopolymer and provides models that are useful for casting or molding as well as for conceptual evaluation.
Photopolymers are well known as liquids that solidify or harden with exposure to specific wavelength and intensity of electromagnetic radiation. This process is called photopolymerization or photohardening. Most commonly used in the field of rapid prototyping are ultraviolet (UV) or electron-beam (EB) curable acrylates and epoxy resins. When the proper radiation strikes the liquid, the top layer of the material will solidify. Just as cold air may solidify water into ice on the surface of a lake in winter, the radiation does not penetrate through the entire reservoir of liquid photopolymer. Control of the exposure time, and thereby the light energy impinged onto the liquid, limits the depth of polymerization to a programmed amount. In RP, the programmed layer effects finish quality of a completed model. The thinnest possible layer thickness leads to a smooth model finish. Careful control of the polymerization is also critical to the accuracy and strength of the finished model.
As radiation impinges the free surface of the liquid photopolymer, it is dispersed and absorbed. This effect limits the penetration of the radiation and thus the depth of hardening. This depth is known as the cure depth, C.sub.d. The cure depth is a function of exposure and therefore, may be controlled by varying the radiation level applied to the imaging surface. In this manner, the layer thickness may be controlled to balance the speed of building and resolution.
Currently marketed and patented designs for stereolithography use light sources to alter the state of a light curable photopolymer. The liquid changes state to a solid form when light, with the correct wavelength and intensity, focuses upon it. The thin solidified layer becomes a sheet like surface that when built upon with many successive layers become a three-dimensional object. The shape of each layer is a cross section of a three-dimensional solid object designed on a computer aided design (CAD) system.
The process of forming three-dimensional objects by photohardening was proposed in U.S. Pat. No. 2,775,758 by Munz. Articles by Kodama in 1981 referenced the use of ultraviolet light for the photohardening of successive layers. Succeeding patents by Hull and Fudim (U.S. Pat. Nos. 4,575,330; 4,752,498; 4,801,477) described apparatuses and methods for layer wise building of models from photopolymer resins.
In a system commercialized by 3D Systems, Inc., of Valencia, Calif., and called a stereolithography apparatus (SLA), radiation energy that is provided by a laser, causes resin solidification. The laser beam is directed by a galvanometer or acousto-optic-modulator (AOM) to trace the cross-sectional image with the focused laser spot. The laminate layers solidified by the laser adhere due to overcure or overlapping of the cure depth. The exposure of the layer must cure deeper than the programmed resin layer thickness. In this manner, the new layer cures into the previous layer and adheres like a lamination. The programmed thickness of a layer must be less than the achieved cure depth in order to insure layer cohesiveness. With this method, a problem occurs. As the photopolymer cures, the volume of solidified material shrinks due to the internal chemical reaction. As a laser traces the image across a liquid surface, areas of the surface layer are at different stages of polymerization resulting in an uneven cure profile. As a new layer attaches to a previous layer, the shrinking of the new layer can cause the previous layer to curl in a bimetallic type affect. This is especially troublesome in large flat areas and creates internal stresses that lead to deformity and weakened model strength. This curl affect may cause the layers to stratify.
As previously mentioned, a light source is controlled to expose a cross-sectional pattern for a programmed exposure time. This process currently assumes two separate embodiments in exemplary systems as SLA and Solid Ground Curing (SGC). SLA provides arguably the higher accuracy of the two systems since the light source is a laser. The focused, intense spot of the laser provides a small, accurate point of light that causes solidification. This has the advantage of exposure limited to a very small area and therefore allows accurate reproduction of the cross-sectional image. One disadvantage of the system is that tracing an entire pattern with a small point of light is time consuming. As previously mentioned, the SLA system also suffers from an uneven cure profile that can lead to curl problems.
The SGC system utilizes an optical mask, generated by ionographic techniques, to reproduce the cross-sectional pattern. A high power UV emitting lamp exposes the entire cross section in a single shot. This greatly reduces the time of exposure for each layer and thus reduces the part build time. However, as previously mentioned, the polymerization process requires careful control of the light exposure. Inaccuracies in the SGC system introduced by the light source cause an uneven cure. Combined with the extensive mechanical systems used to generate the optical mask, this system sees a loss of accuracy. The SGC system compensates with a flying cutter to mill each layer flat as is it produced. This lengthens the time to build each layer and increases the number of mechanical systems. These mechanical support systems lead to high maintenance and loss of repeatability and accuracy over the life of the machine.
Early stereolithography systems coated the model by "deep dipping" below the surface and then returning to the desired level. This system caused problems as the thin, partially hardened layers were moved through the viscous liquid. The forces on the fragile structures caused mechanical inaccuracies. The wiper systems described in U.S. Pat. No. 5,651,934 (Almquist, et al.) succeeded in forming a thin layer with a dipping process that reduced the forces on the part. This system uses a blade to sweep across the surface to form a layer of the desired thickness. This sweeping process still requires a slow, short lowering of the model into the liquid reservoir and then raising it again. While reducing the stresses on the part, this process does put forces on the fragile parts that can lead to inaccuracies.